Cardiorespiratory responses to 6-minute walk test in interstitial lung disease: not always a submaximal test

Background: The 6-minute walk test (6MWT) is used to measure exercise capacity and assess prognosis in interstitial lung disease (ILD). Although the 6MWT is usually considered to be a test of submaximal exercise capacity in ILD, the physiological load imposed by this test is not well described and 6MWT outcomes are poorly understood. This study aimed to compare cardiorespiratory responses to 6MWT and cardiopulmonary exercise test (CPET) in people with ILD. Methods: 47 participants with ILD (27 idiopathic pulmonary fibrosis (IPF), mean age 71 (SD 12) years, diffusing capacity for carbon monoxide (TLCO) 49(15) %predicted) undertook CPET and 6MWT on the same day in random order. Oxygen uptake (VO2), ventilation (VE) and carbon dioxide production (VCO2) were assessed during each test using a portable metabolic cart. Results: The VO(2)peak during the 6MWT was lower than during CPET (1.17(0.27) vs 1.30(0.37) L.min(-1), p = 0.001), representing an average of 94% (range 62-135%) of CPET VO(2)peak. Achieving a higher percentage of CPET VO(2)peak on 6MWT was associated with lower TLCO %predicted (r = -0.43, p = 0.003) and more desaturation during walking (r = -0.46, p = 0.01). The VEpeak and VCO(2)peak were significantly lower during 6MWT than CPET (p < 0.05). However, participants desaturated more during the 6MWT (86(6)% vs 89(4)%, p < 0.001). The degree of desaturation was not affected by the percent of peak VO2 achieved during the 6MWT. Responses were similar in the subgroup with IPF. Conclusions: On average, the 6MWT elicits a high but submaximal oxygen uptake in people with ILD. However the physiological load varies between individuals, with higher peak VO2 in those with more severe disease that may match or exceed that achieved on CPET. The 6MWT is not always a test of submaximal exercise capacity in people with ILD.